1. Field of the Invention
This invention relates to a new and improved watthour meter damping assembly and more particularly to such an assembly including a pair of highly coercive and anisotropic permanent magnets extending in opposing relationship from the ends of a magnetic yoke and having a temperature compensator uniquely extending between the permanent magnets.
2. Description of the Prior Art
Induction electromechanical watthour meters generally include an electromagnetic unit having voltage and current sections connected to an electrical load for measuring the consumption of AC electric energy. AC magnetic fluxes from the electromagnetic unit produce a driving torque on a rotatable electroconductive meter disc by interaction with eddy currents induced in the disc by the fluxes. Another general and essential part of a watthour meter includes the magnetic brake or damping assembly. A unidirectional or DC braking magnetic flux provides a retarding torque on the disc which is proportional to the disc speed. The required retarding torque balances the driving torque so that each rotation of the disc is proportional to a predetermined number of watthours of electrical consumption.
The braking magnetic flux is typically provided by permanent magnets which direct the flux through an air gap space receiving the meter disc. The strength of the permanent magnets, the position of the braking magnetic flux in relation to the disc center, the area and density of the braking flux entering the disc, and the length of the air gap spacing determines the amount and consistency of the retarding torque. Maintaining the braking magnetic flux constant is a chief requirement. Eddy currents induced by the permanent magnet fluxes interact with the fluxes to oppose the driving torque in a so-called "square law" relationship. The flux decreases in proportion to the square of the length of the air gap in accordance with this relationship. It is essential that retarding torque that is produced by the damping assembly be kept proportional to the speed of the meter disc for accurate meter measurements.
Thus, the damping assembly air gap must be kept at a constant minimum spacing when the flux is constant for efficient operation over very long meter lifetimes, while being subject to widely varying temperature and atmospheric changes and sometimes to substantial shock and vibration during handling and shipping. Also, when using high energy product permanent magnets such as those made with cobalt-rare earth magnet materials, the high coercivity and highly anisotropic magnet characteristics tend to pull the magnets together during assembly when they are oriented in an opposite facing relationship to define the air gap space. Reductions in the air gap due to the mutual magnetic attractions must be avoided to further maintain stable and accurate meter calibrations.
The damping assemblies also require an adjusting feature to properly calibrate the retarding torque. This adjustment is referred to as a full load adjustment in the watthour meter art. Such adjustments are provided either by changing the position of the braking magnetic flux with respect to the center of the disc or by varying the amount of the air gap flux by means of a magnetic shunt. The former method requires positioning to be made with respect to two effects in which positioning the braking flux further away from the disc center increases the lever-arm effects so as to increase the retarding torque and the other effect increases the retarding torque as increases incur in the rate at which the disc cuts the braking magnetic flux. In many magnetic braking assemblies two or double air gaps are employed and the positioning of these must be made with respect to the above two effects and also with respect to the retarding torque effects of each air gap flux with respect to the other. The second method of adjustment usually includes the use of a soft magnetic screw which varies the reluctance of the flux return path for the permanent magnet fluxes.
Temperature compensation of magnetic damping assemblies is a further essential requirement so that the braking magnetic flux through the air gap is kept constant. The permanent magnet materials typically used in damping assemblies have a negative temperature coefficient so as to decrease in strength with increase of temperature. In the field of watthour meters this compensation is referred to as Class I temperature compensation. A temperature responsive magnetic shunt such as one having a negative temperature coefficient of permeability is typically employed so as to divert larger amounts of the air gap flux at low temperatures and to divert lesser amounts at high temperatures. Proper positioning, size, shape of the temperature compensator and proper selection of the compensator material characteristics are required. Temperature compensators are typically positioned adjacent to and in parallel (shunting) relationship with the direct paths of the braking magnetic fluxes to thereby be positioned in the stray or leakage flux paths surrounding the air gap. With highly directional and anisotropic types of permanent magnet material, such as the cobalt-rare earth magnetic materials, the prior temperature compensator positions adjacent the main flux paths are often not sufficiently effective in capturing and effecting changes in the magnetic fluxes. Also, it is especially desirable to arrange the magnetic damping assemblies so that they are most efficient by minimizing the amounts of leakage and stray fluxes which are generally inherently associated with the high reluctance path of an air gap.
A number of the aforementioned characteristics and requirements of watthour meter damping assemblies are disclosed in prior art patents noted hereinafter. In U.S. Pat. Nos. 1,843,518; 2,309,414; 2,832,932; 3,173,067; and German Pat. No. 804,694 permanent magnet damping assemblies are described including U-shaped magnetic yokes for providing a main flux return path for fluxes passing through opposing magnet pole faces defining an air gap. The U.S. Pat. No. 3,173,067 disposes a temperature compensator forming one of the pole face areas for changing the distribution of the braking magnetic flux rather than shunting a portion thereof from the air gap. In U.S. Pat. Nos. 2,309,414 and 2,832,932 stray flux portions of the permanent magnet fluxes are shunted by temperature compensators attached to the ends or sides of the permanent magnets having pole faces directing fluxes through the air gaps. In the German Pat. No. 804,694 thermally expandable temperature compensators are disclosed for changing the air gap spacing with changes in temperature.
U.S. Pat. No. 4,030,031 discloses a damping assembly having double air gaps and using a pair of highly coercive anistropic permanent magnets made of a cobalt-rare earth magnet material.
U.S. Pat. Nos. 1,722,756; 1,734,199; 1,945,523; 2,605,301; and 3,054,953 disclosed single U- or C-shaped magnetic compensator shunts provided for diverting stray or leakage flux from the damping assembly air gaps. All of the last named patents include shunts made of a magnetic material having a negative coefficient of permeability and positioned on the side or sides of permanent magnet pole pieces for increasing the shunting flux at low temperatures and decreasing the shunting flux at high temperatures. In the U.S. Pat. No. 1,734,199 a U-shaped magnetic compensator arrangement is positioned on the sides of the permanent magnet to vary the shunted flux or fluxes in response to the speed of rotation of the meter disc and not in response to temperature.